# Discovery of intrinsic ferromagnetism in 2D van der Waals crystals

## TL;DR

This paper reports the first experimental discovery of intrinsic long-range ferromagnetism in pristine 2D Cr2Ge2Te6 van der Waals crystals, demonstrating tunable transition temperatures and potential for spintronic applications.

## Contribution

It provides the first evidence of intrinsic ferromagnetism in 2D vdW crystals and reveals how small magnetic fields can significantly enhance transition temperatures.

## Key findings

- Intrinsic ferromagnetism observed in 2D Cr2Ge2Te6
- Transition temperature can be increased by 35-57% with small magnetic fields
- Field dependence of transition temperature is a hallmark of 2D soft ferromagnets

## Abstract

It has been long hoped that the realization of long-range ferromagnetic order in two-dimensional (2D) van der Waals (vdW) crystals, combined with their rich electronic and optical properties, would open up new possibilities for magnetic, magnetoelectric and magneto-optic applications. However, in 2D systems, the long-range magnetic order is strongly hampered by thermal fluctuations which may be counteracted by magnetic anisotropy, according to the Mermin-Wagner theorem. Prior efforts via defect and composition engineering, and proximity effect only locally or extrinsically introduce magnetic responses. Here we report the first experimental discovery of intrinsic long-range ferromagnetic order in pristine Cr2Ge2Te6 atomic layers by scanning magneto-optic Kerr microscopy. In such a 2D vdW soft ferromagnet, for the first time, an unprecedented control of transition temperature of ~ 35% - 57% enhancement is realized via surprisingly small fields (<= 0.3 Tesla in this work), in stark contrast to the stiffness of the transition temperature to magnetic fields in the three-dimensional regime. We found that the small applied field enables an effective anisotropy far surpassing the tiny magnetocrystalline anisotropy, opening up a sizable spin wave excitation gap. Confirmed by renormalized spin wave theory, we explain the phenomenon and conclude that the unusual field dependence of transition temperature constitutes a hallmark of 2D soft ferromagnetic vdW crystals. Our discovery of 2D soft ferromagnetic Cr2Ge2Te6 presents a close-to-ideal 2D Heisenberg ferromagnet for studying fundamental spin behaviors, and opens the door for exploring new applications such as ultra-compact spintronics.

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Source: https://tomesphere.com/paper/1703.05753